The present invention relates to an apparatus for controlling a converter which is constituted by self-arc-extinction elements (hereafter defined as GTO) such as a gate turn-off thyrister and connected between a load and a power system through a transformer, and more particularly to a converter controlling apparatus capable of preventing a direct current magnetic deflection in the transformer.
FIG. 1 is a block diagram showing an apparatus for controlling a conventional converter (hereafter called as a self-commutation converter) including the GTO. In the figure, numeral 1 denotes a power system, 2 is a transformer for connecting the power system with a self-commutation converter 3, 4 is a direct current source such as a capacitor, 5A is a current detector for measuring a current of a primary coil in the transformer, 5B is a current detector for measuring an output current of the self-commutation converter 3, 6 is a potential transformer (PT) for measuring a voltage of the power system 1, 7 is a voltage/current control circuit for controlling a voltage and current in the system according to a system voltage reference 51 and an output current reference 52, 8 is a direct current component detector for detecting a direct current component included in the output current of the self-commutation converter 3 and the current which is measured by the current detector 5B, 9 is an adder for adding an output of the voltage/current control circuit 7 with an output of the direct current component detector 8, 10 is a pulse width modulation (PWM) control circuit for determining an ignition timing of the GTO corresponding to an output of the adder 9 and for regulating an output voltage, and 11 is a gate pulse amplifying circuit.
In FIG. 1, the PWM control circuit 10 determines a ignition pattern of the GTO in the manner that the direct current component is not included in the output voltage of the self-commutation converter 3. However, the actual output voltage has a waveform including a direct current component because of a characteristic of the GTO and an unevenness of a transmission time of a gate signal. When the direct current component is included in the output voltage of the self-commutation converter 3, an iron core of the transformer 2 is magnetically deflected because a product of time by voltage supplied to the transformer 2 per one cycle does not become to zero to increase an excitation current to be over, thereby stopping a protection of the self-commutation converter 3.
In the worst case, the situation results a destroy of the elements constituting the converter. For preventing a magnetic deflection in the conventional circuit shown in FIG. 1, the current detector 5B detects the output current of the self-commutation converter 3, the direct current component detector 8 detects the direct current component occurring in a step of reaching to the magnetic deflection, and the adder 9 adds the direct current component with an output voltage reference value from the voltage/current control circuit 7 to perform the PWM control, thereby regulating the output voltage of the self-commutation converter 3 in the manner of cancelling the magnetic deflection.
Even though the voltage of the power system 1 is an alternating current (AC) voltage, a direct current (DC) component is transitionally included in the voltage of the power system, for example, when a power capacitor or the transformer is switched on. However, even though the conventional control apparatus can correct the direct current component outputted from the self-commutation converter 3, the apparatus can not suppress the magnetic deflection by the direct current component generated by the power system 1 because the direct current component occurring in the power system is not corrected. Accordingly, when the direct current component is transitionally generated, the transformer 2 is magnetically deflected to reach to an over-current.